This invention relates in general to improvements in separation processes for the physical separation of different species of the material constituents of a mixture of species, more particularly to new methods and means for increasing the respective concentrations of separated species of such constituents. The invention is applicable to a wide variety of physical mixtures, such as separating ice crystals from pulverized, frozen aqueous solution, as well as to the beneficiation of ores. It has been found to be particularly useful in the separation of impurities form coal, i.e.: coal benefication.
The present invention relates broadly to separation of dissimilar species ranging in size from macroscopic particles to molecular mixtures. Recent advances in biotechnology have allowed production of hormones, enzymes, antibodies and other biologically active materials, and large scale production of these will revolutionize the treatment of disease, the raising of plants and animals for food, and the renewable synthesis of industrial materials. Separation technology has not kept pace with the needs of the biotechnology industries. Separation and purification of these materials is the most costly and difficult aspect of making these products on a preparative scale. Scale up of laboratory techniques has allowed production, but at high cost. A low cost, effective separation technique would lower the cost of these products and enormously increase the benefits that will result.
Laboratory analytical techniques emphasize sensitivity and the detection of very low levels of material. Techniques such as chromatography, field flow fractionation (FFF) (see Giddings U.S. 3,449,938), electrophoresis, micro-sieving, molecular distillation, and liquid-liquid extraction originated in the laboratory and have limited commercial application. These separation techniques are expensive when scaled to produce commercial quantities of materials requiring high levels of separation.
Attempts have been made to scale laboratory techniques using apparatus of commercial size. Problems arise because of the difficulty of maintaining consistent flow over a large cross section. In preparative scale chromatography one way to achieve consistent flow is to use a packing with a large pressure drop, but channeling and flow maldistribution can still occur. In preparative eletrophoresis, convection currents from heating due to the applied current can disrupt the separation to such an extent that performing the separation in zero gravity in orbit is expected to be economic for some separations. The state of the art of electophoresis practice is well described in Electrophoresis '86, Proceedings of the Fifth Meeting of the International Electrophoresis Society, M. J. Dunn, ed., VCH Publishers, 1986. Chromatography can be considered to be countercurrent liquid-liquid extraction where one of the liquids is a stationary phase held on the stationary support. The stationary liquid phase never leaves the separation column and so is subject to eventual degradation, necessitating replacement. The stationary phase renders chromatography inherently a batch-type process.
Distillation uses the pressure difference between boiler and condenser to provide the driving force for moving vapor countercurrent with respect to the liquid. Many compounds with low vapor pressure are degraded at temperatures high enough to provide a vapor pressure sufficient to move the vapor at an effective mass flow rate.
The classification of particles by size is often achieved through sieving. Maintaining the dimensional accuracy of the sieve, preventing the sieve from becoming clogged, and the necessarily thin and delicate construction of fine mesh screens presents problems in operation that have limited screening practice to coarse particles.
Liquid-liquid extractin is a very powerful separation technique, but is difficult to practice when the two fluids have similar densities, high viscosities and low interfacial tension. Such fluids are commonly used to separate proteins, cell components and DNA. Aqueous polymer mixtures of different molecular weight form near-critical two phase systems that can be used to separate biological material. The state-of-the-art is well described in a book Partitioning in Aqueous Two-Phase Systems, edited by H. Walter et al. Academic Press, 1985. Gravity and centrifugal effects have been used to move and separate the two phases, but this is a time consuming process, and limits the number of countercurrent stages that are practical.
The constituents of coal which are considered to be "impurities" include those containing sulfur and some minerals which form non-combustible ash. Ash-forming constituents coat, foul and reduce the efficiency of heat transfer in boilers in addition to polluting the environment. Sulfur-bearing constituents contribute to environmental pollution, one form of such pollution being commonly referred to as "acid rain". As found in its natural state, coal contains varying proportions of these impurities, the proportions in any one deposit depending on the geological history of that deposit.
This invention teaches new methods and means for electrically charging and separating different species of the constituents of coal and other ores, solutions and slurries, including power-like ultra-fine particles sizes (e.g.: smaller than 100 microns), and for electrically charging a mixture which includes such ultra-fine particles, so as to enable particles of impurities and particles of coal, phosphate, solute or other desired component, or species of constituents of any such mixture, to be separated from each other in an electric field more efficiently than has heretofore been achieved on a commercial scale.